When you admire things or people, you look at them or think about them with wonder, appreciation, or respect. In other words, to admire something is to think it's beautiful, special, or otherwise great in some way.
You might stand in a national park and admire the view.
If you admire your grandparents, that means you respect them and look up to them; you believe they're wonderful people with good hearts who have done great things in their lives. Your grandparents are admirable. And they deserve your admiration.
At Bilbo's 111th birthday party, he tells his hobbit friends, "Eleventy-one years is far too short a time to live among such excellent and admirable hobbits." He means that he admires his friends: he believes they are good-hearted, noble, and kind.